A laser machining system, in which a plurality of processing heads share a common part carrier (“chuck”), may exhibit a head-to-head offset along the direction of chuck travel. A head-to-head offset is a misalignment between the processing heads in the direction of chuck travel. Failing to correct a head-to-head offset may result in degradation of the laser machining system's accuracy and performance.
FIG. 1 illustrates a two-head laser processing system 100 with a head-to-head offset 160. The system 100 includes a chuck 142 that moves a workpiece 143 in the direction of the Y-axis (as indicated by arrow 138). The chuck is shared by a processing head 126 and a processing head 130, which may concurrently process the workpiece 143. The processing heads 126, 130 are connected to an X-axis beam 132. The processing heads 126, 130 may move independently along the X-axis beam 132, in the direction of the X-axis (as indicated by arrows 134, 136). The processing heads 126, 130 emit laser beams 120, 128. Each processing head 126, 130 is optically associated with a focusing lens 112, 110 that focuses a respective incident laser beams 128, 120 on the workpiece 143. As illustrated, the head-to-head offset 160 is a misalignment of the laser beams 128, 120 in the direction of chuck travel. Because the chuck 142 is shared between the processing heads 126, 130, the head-to-head offset 160 may not be corrected by repositioning the chuck 142.
There are at least three common ways of addressing a head-to-head offset in a laser processing system: 1) the offsets are measured, and the chuck is commanded to move to an “average” position that minimizes the maximum deviation from the desired location for any one head; 2) the offsets are measured, and then eliminated as much as possible by adjusting the position of one or both processing heads in the direction of chuck travel (e.g., by using shims or set-screws); or 3) in the case of laser processing systems that include a secondary beam positioner (such as a tip-tilt mirror or a pair of galvanometers) for each processing head, the offsets are measured and compensated for by the secondary beam positioner. There are substantial problems with the three standard approaches outlined above for correcting a head-to-head offset. The details of the three standard approaches are illustrated in FIGS. 2, 3, and 4.
FIG. 2 illustrates a prior art approach for minimizing the error introduced by a head-to-head offset. This approach “splits the differences” of a head-to-head offset 260 between the processing heads 226, 230 by commanding a chuck 242 to move to an average or “compromise” position. Using this approach, the chuck 242 is positioned such that two target feature locations 270, 274 are along a line 262 that is at the midpoint of the head-to-head offset 260. As will be appreciated, the two processing heads 226, 230 cannot create the features at the target feature locations 270, 274 because of the head-to-head offset 260. Accordingly, the distance between the actual feature locations 272, 276 and the target feature locations 270, 274, respectively, is half of the total head-to-head offset 260. While this approach minimizes the worst-case feature placement error introduced by a head-to-head offset 260, this approach does not improve the spread of feature placement error, which remains equal to the total head-to-head offset 260.
FIG. 3 illustrates another prior art approach for correcting a head-to-head offset 360 between the processing heads 326, 330 by adjusting the position of a processing head 330 in the direction of chuck travel. In this approach, the processing head 330 is moved from a first position to a second position (shown as repositioned processing head 330′ in phantom lines). The repositioning of the processing head 330 is in the direction of chuck travel (the Y-axis direction), and may thus compensate for the head-to-head offset 360. In other words, the repositioned processing head 330′ may be aligned with the processing head 326. The processing head 330 may be repositioned by using shims or set screws. While this approach corrects the head-to-head offset 360, designing a processing head that allows for repositioning along the direction of chuck travel may be difficult, and the procedure for correcting the head-to-head offset 360 by repositioning the processing head 330 may also be difficult and time-consuming. A processing head that can be repositioned with respect to the X-axis beam 332, may not be as secure as a processing head that is permanently attached to the X-axis beam 332. This degraded stage stiffness may introduce vibration into the system when the processing head is moved. Finally, set screws or shims may move over time, which may cause the head-to-head offset to return.
FIG. 4 illustrates another prior art approach, where one or more secondary beam positioners 480, 485 are used to compensate for a head-to-head offset 460. Two processing heads (not shown) may be optically associated with the secondary beam positioners 480, 485. Each processing head may emit an incident laser beam 420, 428. The secondary beam positioners 480, 485 may each include a pair of galvanometers 481, 482 and 486, 487 connected to beam steering mirrors 483, 484 and 488, 489, respectively. The secondary beam positioners 480, 485 allows the incident laser beams 420, 428 to be quickly steered within respective limited scan fields 490, 492. The secondary beam positioners 480, 485 enable “fast” laser beam steering because the laser beams 420, 428 may be repositioned without moving the processing head (not shown) or the chuck 442. As illustrated, the secondary beam positioner 485 may be positioned so as to eliminate the head-to-head offset 460. This approach, however, requires sacrificing a portion of the limited scan field 492 associated with the secondary beam positioner 485. Only a portion 491 of the total limited scan field 492 may be used when the secondary beam positioner 485 is used to correct a head-to-head offset 460. While this approach may be tolerable in cases where the head-to-head offset is small in relation to the total limited scan field 492, this approach imposes additional limitations. For example, in laser machining systems that use assist gas flow that is substantially coaxial with the processing laser beam, the limited scan field may already be severely restricted because of a nozzle with a small orifice to direct the assist gas flow. In such systems, there may not be a substantial portion within the limited scan field to sacrifice for head-to-head offset compensation purposes.